Ethylene is an important primary feedstock in the petrochemical industry. In certain areas of the world, exemplary of which is Alberta, Canada, the petrochemical industry relies upon the occurrence of ethane in natural gas as its main source of ethylene. Typically, such gas would contain from two to four percent ethane content, which ethane is usually extracted at the well head and thereafter converted to ethylene. In recent years, it has been observed that the produced natural gas is becoming depleted in ethane.
Methane, thermodynamically the most stable hydrocarbon, occurs in abundance in natural gas. It would be highly desirable, therefore, to be able to effect the conversion of methane to ethylene, thereby providing an abundant source of the latter.
However, such a conversion is highly endothermic; under the high temperature reaction conditions required therefor, control of the reaction to prevent the formation of unwanted by-products is difficult.
Heretofore, the techniques tried to effect the cracking of methane have ranged from thermal techniques to low and high frequency electrode and electrodeless discharge, triboelectric discharge, plasma jets and laser irradiation.
It is generally accepted that the major products in the electric discharge decomposition of methane are H.sub.2, C.sub.2 H.sub.6, C.sub.2 H.sub.4, and C.sub.2 H.sub.2. The relative amounts of these products are usually dependent upon the experimental conditions. For example, McCarthy (R, L. McCarthy, J. Chem. Phys., 22, 1360 (1954) ) reported that in a glow discharge of methane at 150-310 V/cm and at a CH.sub.4 pressure of about 16 torr and residence time less than 10 s, the principal product was acetylene. Ethylene and ethane, however, were only observed as major products when the discharged methane gas was allowed to impinge directly on the wall of a cold trap at 77 K. These findings provided the evidence that free radicals, such as the CH.sub.3, CH.sub.2 primary radicals do not recombine instantly under such conditions but persist for some time after leaving the plasma zone.
A number of electric arc cracking of methane studies have been made by Du Pont and others giving a % conversion of methane ranging from 50 to 80. In pilot-plant experiments, Eremin (E. N. Eremin and co-workers, Zh. Fiz. Khim., 37 1487 (1963) ) obtained the best conditions for the best results in the electric discharge of methane at a pressure of 40 torr and a power consumption of 3kWHr m.sup.-3 (10.sup.3 kJ liter.sup.-1 ) which showed 60% of the methane was cracked and 15% C.sub.2 H.sub.2 was formed.
In microwave discharge experiments, the best energy yield was reported by McCarthy; it was approximately 3600 kJ for one mole of C.sub.2 hydrocarbon produced.
All of the previous methane cracking experiments have been limited to low pressure plasma/electric discharge decompositions, where the reactions presumably occurred in the gas phase. Apart from the initial cost of the high power apparatus, the energy requirements for cracking the methane under such conditions are excessive.
There is therefore a need for a simple, inexpensive process which can convert methane to ethylene in good yield.